1. Field of the Invention
The present invention relates to a piezoelectric vibrator having a main vibrator body formed of piezoelectric material, and to a vibratory gyroscope whose operation is based on the detection of Coriolis force acting on the piezoelectric vibrator in a rotating system, and particularly to a piezoelectric vibrator for producing a large vibration amplitude signal while consuming a small amount of electric power, and to a vibratory gyroscope using this piezoelectric vibrator.
2. Description of the Related Art
A first conventional piezoelectric vibrator used in vibratory gyroscopes typically includes an elastic member, formed from an elastic alloy such as elinvar, and piezoelectric elements glued to the surfaces of the elastic member. The elastic member is vibrated by applying an alternating current (ac) voltage across the piezoelectric elements, thereby causing vibratory deformation of the piezoelectric elements which results is vibration of the elastic member. However, these first conventional piezoelectric vibrators have a poor driving efficiency due to frictional losses during the energy transfer between the piezoelectric elements and the elastic member. Because of this poor driving efficiency, it is necessary to apply an undesirably high ac voltage to the piezoelectric elements in order to vibrate the elastic member at a desired vibrational amplitude.
A second conventional piezoelectric vibrator avoids the problems associated with the first conventional vibrator by forming the vibrator entirely from a piezoelectric material, such as a piezoelectric ceramic. This produces a better driving efficiency and yields a larger strain from a lower application voltage as compared with the above-mentioned first conventional vibrator.
FIGS. 8A and 8B show front surfaces of an example of the second conventional piezoelectric vibrator formed from piezoelectric ceramic. A main vibrator body 1 has a flat shape and includes a cantilever structure which extends in the z-axis direction (normal to the drawing). Formed on the top and bottom surfaces 1a and 1b of the main vibrator body 1 are electrodes 2a, 2b and 2c which also extend in the z-axis direction. To polarize the main vibrator body 1, a positive voltage is applied to electrodes 2a, a negative voltage is applied to the electrodes 2c, and the middle electrodes 2b are grounded. The resulting electric fields between the electrodes 2a and 2b on both surfaces 1a and 1b produce a dielectric polarization which is depicted by arrows provided in FIGS. 8A and 8B, and a similar dielectric polarization is also produced between the electrodes 2b and 2c.
In operation, to vibrate the main vibrator body 1 in the x-axis direction, an ac driving voltage (such as that shown in FIG. 7A) is applied to the electrodes 2a and 2c in the same phase relation while the middle electrode remains grounded, as shown in FIG. 8B. When a positive voltage is applied to the electrodes 2a and 2c, at a particular point in time, sections of the main vibrator body 1 (which are marked "O" in FIG. 8B) have a positive strain (expansion) and sections marked "x" have a negative strain (contraction), causing the main vibrator body 1 to bend in the +x direction with respect to the neutral plane O--O. When the ac driving voltage applied to the electrodes 2a and 2b is reversed (negative), the main vibrator body 1 bends in the -x direction. Thus, application of the ac driving voltage causes the main vibrator body 1 to vibrate (repeatedly bend) in the -x and +x directions (the x-axis direction).
Piezoelectric vibrators of the above-mentioned second conventional type are used in various kinds of instruments. For example, in a vibratory gyroscope, when the main vibrator body 1 is rotated around the z axis while being vibrated in the x-axis direction, vibrational components in the y-axis direction are caused by Coriolis force. The main vibrator body 1 therefore has a motion which is a combination of the driving vibration in the x-axis direction and the vibrational components in the y direction caused by Coriolis force. By extracting the vibration component in the y-axis direction caused by the Coriolis force in the form of voltage changes through the electrodes, the angular velocity of the gyroscope can be detected.
The piezoelectric vibrator having its main vibrator body 1 formed from piezoelectric material, as shown in FIGS. 8A and 8B, has a better driving efficiency than that of the above-mentioned first conventional piezoelectric vibrator in which the strain of piezoelectric material is propagated to the constantly-elastic material.
However, the prior art piezoelectric vibrator shown in FIGS. 8A and 8B produces dielectric polarization only between the electrodes aligned on each of the top surface 1a and bottom surface 1b of the main vibrator body 1. Therefore, the positive strain indicated with the symbol "O" and negative strain indicated with the symbol "x" created on the surfaces 1a and 1b in response to application of the ac driving voltage produce a relatively small bending moment in the x-axis direction. Specifically, the positive strain and negative strain created at a particular point in time between the electrodes 2a and 2b and between the electrodes 2b and 2c on the top surface 1a are localized at the surfaces 1a and 1b, and therefore these strains do not produce a large bending moment around the neutral plane O--O.
Accordingly, improvements in the driving efficiency of the second conventional piezoelectric vibrator over that of the first conventional piezoelectric vibrator is limited. That is, in order to drive the main vibrator body 1 to produce a large vibration amplitude in the x-axis direction, it is necessary to apply a high ac driving voltage to the electrodes 2a and 2b. Therefore, it is not possible to produce large-amplitude vibrations in response to an ac driving signal which consumes a small amount of power.